The present invention relates to a filter and a filter medium to be used in said filter, which can be effectively used in primary filtration for purification of industrial water or industrial waste water or for food industry including beverage industry, i.e. in a applications where there are too many impurities in a liquid and the use of filter cartridge is neither economical nor practical because the cartridge must be replaced too frequently.
For filtration in the applications for recycling of industrial water, a sand filter using sand, anthracite, etc. as filter media and a diatomaceous filter are used. In the diatomaceous filter, diatomaceous earth is coated on a filter medium of a filter cartridge using a metal screen etc. as the filter medium. After coating the diatomaceous earth, liquid to be purified is introduced. In recent years, as an alternative for the filter media such as sand, anthracite, etc., a modified sand filter which uses mop-like fibers or aegagropila-like fibers is propagated. In these types of filters, when the filter media become clogged, they are replaced in such manner that filter meshes are restored to the initial condition to some extent by backwashng using the raw liquid to be filtered and the filter media are replaced when they cannot be restored any more.
On the other hand, since the cost of the raw liquid itself to be used for backwashing is expensive in the food industry, there have been difficulties with the restoration of filter media by backwashing in terms of cost because the cost for bringing out the raw liquid for backwashing is too extensive. Therefore, when a filter cartridge in the filter used for such applications, becomes clogged it is general practice to replace it by a new filter cartridge. Although this filtration technique is a time-honored technique, a bag filter has been developed by technical progress in filter media in recent years, which uses a bag-shape filter cloth with a density gradient and made of fine fibers. Although the bag filter is now widely used, it must also be replaced with new bag filter when clogged.
There are many problems in conventional type filtration methods as described above, and it appears that none of these methods are completely satisfactory.
In the case of backwashing of a sand filter, there are problems not only with the cost involved in the use of a backwashing liquid as explained above, but also with the costs for processing such as cost of processing tanks, ultrafiltration (UF), reverse osmotic membrane filtration (RO), etc., and further, the processing cost of final waste liquid. If the problems of the processing costs are neglected, waste liquids are discharged thoughtlessly, and the problem of environmental pollution may occur.
In case of a filter cartridge, the problem of processing cost of concentrated liquid does not occur, but there are problems such as high cost due to the structure of filter cartridge itself, high frequency replacement of the cartridge filter and the outflow of foreign objects to secondary side when filter is replaced.
The bag filter is now widely used in recent years because the problem of the high cost of filter, i.e., the most serious problem of filter cartridge system, can be eliminated or foreign objects do not flow out to secondary side and can be processed as they remain in bag. However, a problem lies in the complicated procedure to replace bag.
Further, there is a conventional method which introduces raw liquid to a wind-up type sheet-like filter medium and which performs self-weight filtration. Because this filtration is by self-weight, unless the filtration tank is designed deeper to increase the applied liquid pressure, overflow may occur even when filter media are slightly clogged if they are not wound up. For this reason, it is not widely used in the field of precision filtration.